1. Field of the Invention
This invention relates to a carrier used in a developer for developing electrostatic latent images in electrophotography, electrostatic recording, electrostatic printing or toner jet recording, a two-component developer having the carrier and a toner, and an image forming method making use of the two-component developer.
2. Related Background Art
A number of methods are known as methods for image forming processes. In particular, it is common to use an image forming method like the following: First, an electrostatic latent image is formed on a photosensitive member by various means utilizing a photoconductive material. Subsequently, the latent image is developed by the use of a toner to form a toner image as a visible image. Then, the toner image is transferred to a recording material such as paper as occasion calls, and thereafter the toner image is fixed by the action of heat and/or pressure to obtain a copy. The toner that has not transferred to and has remained on the photosensitive member is removed by cleaning by various methods, and then the above process is repeated.
In recent years, image forming apparatus making use of such an image forming method have severely been in pursue toward smaller size, lighter weight, higher speed and higher reliability. Also, such image forming apparatus not only have been used merely as copying machines for office working to take copies of originals as commonly done, but also have begun to be used as digital printers for outputting data from computers or used for copying highly minute images such as graphic designs.
As to the step of cleaning the photosensitive member, cleaning means such as blade cleaning, fur brush cleaning and roller cleaning have conventionally been used. Such cleaning means are those by which the transfer residual toner on the photosensitive member is scraped off or blocked up so that it can be collected in a waste toner container. Hence, because of the fact that the member constituting such a cleaning means is brought into pressure touch with the surface of the photosensitive member, problems have tended to arise. For example, bringing a cleaning member into strong pressure touch causes the surface of the photosensitive member to wear. Moreover, the whole apparatus must be made larger in order to provide such a cleaning means. This has been a bottleneck in attempts to make apparatus compact. In addition, from the viewpoint of ecology, a system that may produce no waste toner is long-awaited.
To solve the above problems, an image forming apparatus is proposed which employs a technique called “cleaning-at-development” (cleaning performed simultaneously at the time of development) or “cleanerless” (see, e.g., Japanese Patent Publication No. H05-69427). In this image forming apparatus, one image is formed at one rotation of the photosensitive member so that any effect of transfer residual toner does not appear on the same image. A technique is also proposed in which the transfer residual toner is dispersed or driven off by a drive-off member to make it into non-patterns so that it may hardly appear on images even when the surface of the same photosensitive member is utilized several times for one image (see, e.g., Japanese Patent Applications Laid-open No. S64-20587, No. H02-259784, No. H04-50886 and No. H05-165378).
As a need of users in these days, it is also sought to achieve higher image quality and higher minuteness. As a means for achieving it, what prevails is to make toners into fine particles. Making toners into fine particles is certainly greatly effective in the sense that latent images are faithfully reproduced. However, fog must be remedied in order to provide stable images over a long period of time. This is namely because making toners have a smaller particle diameter makes the toners have a larger particle surface area, resulting in a broad charge quantity distribution, and this tends to cause fog. Making toners have a larger particle surface area makes charge characteristics of toners more tend to be influenced by environment. Further, making toners have a small particle diameter makes the state of dispersion of a charge control agent or a colorant have great influence on the chargeability of toners. When such toners having a small particle diameter are used in high-speed machines, excess charging may result especially in an environment of low humidity to cause fog or a decrease in density.
Where such toners having a small particle diameter are used, faulty cleaning tends to occur in a system in which the transfer residual toner on the photosensitive member is removed by cleaning. On the other hand, in the above cleanerless system, transfer residual toner due to fogging toner may increase, and its presence may inhibit the photosensitive member from being charged at its charging portions, to cause fog more seriously, making it difficult to provide high-grade images.
As measures to achieve higher image quality and environmental measures to be taken so as not to generate ozone, a system of contact charging such as roller charging has become prevalent as a method of charging the photosensitive member, in place of conventional corona charging. In an aspect of image quality, in the case of the corona charging, it has come about that the toner having scattered tends to contaminate charging wires to make their discharge insufficient at the areas thus contaminated, and this makes it difficult for the photosensitive member (drum) to be provided with a stated potential. Hence, an image defect called line images tends to occur in the contact charging system as well, a charging system in which an alternating current is applied superimposingly on direct-current charging is employed from the viewpoint that the photosensitive member which is a latent image baring member is to be sufficiently charged over a long period of time.
The charging system in which an alternating current is applied superimposingly on direct-current charging can maintain high image quality over a long period time. However, because of the application of an alternating current component, the toner having been interposingly present at the charging portions tends to adhere strongly to the charging member or the photosensitive member. Where the toner has adhered to the photosensitive member, it comes to what is called toner melt adhesion. Where the toner has adhered to the charging member, it causes faulty-charging. Both the cases result in image defects corresponding to the areas to which the toner has adhered. Thus, although various proposals have been made as systems, some problems still remain unsolved in regard to developers suited for the matching with such systems.
Two-component developers, the carrier of which has the function to agitate, transport and charge the toner, are functionally separated as developers, and hence characterized by, e.g., having a good controllability. Accordingly, they are in wide use at present. In particular, they are preferably be used in full-color image forming apparatus such as full-color copying machines and full-color printers, for which a high image quality is demanded.
As magnetic carriers used in the two-component developers, an iron powder carrier, a ferrite carrier and a magnetic-material dispersed resin carrier in which fine magnetic-material particles are dispersed in a binder resin are known in the art. As to the iron powder carrier among these, the carrier has so low a specific resistance that electric charges of electrostatic latent images may leak through the carrier to disorder the electrostatic latent images to cause image defects. Accordingly, as magnetic carriers, the ferrite carrier and the magnetic-material dispersed resin carrier are in wide use at present.
The magnetic-material dispersed resin carrier has advantages such that it has a small specific gravity and can lower agitation torque. Lessening damage to the carrier at the time of agitation prevents carrier-spent from occurring. However, although the developer can be made to have a long lifetime, such a carrier may have an insufficient uniformity in magnetic properties. Hence, this has tended to cause carrier adhesion to tend to cause image defects.
In conventional ferrite carriers, heavy-metal-containing ferrite carriers have commonly been used. In that case, however, the carrier has so large a specific gravity and further has so large a saturation magnetization that it may provide a rigid magnetic brush to have tended to cause deterioration of the developer, such as carrier-spent and deterioration of external additives of toner, and also cause sweep marks of the magnetic brush. Accordingly, light-metal ferrite carriers aiming at lowering specific gravity are disclosed (see, e.g., Japanese Patent Applications Laid-open No. 2001-154416, No. H07-225497 and No. H07-333910). However, these are all ferrite carriers constituted of only light metals, and their constituents have a poor mutual adhesion to have an insufficient particle strength. In particular, in the cleanerless system, a large stress is applied to the developer at the time of agitation in order to secure a good rise of toner charging. Hence, irregular-shape particles tend to be present to tend to cause faulty images.
In addition, in the conventional ferrite carriers, how to manage their particle surface properties is given as a point of concern. Hitherto, a method has been employed in which firing temperature at the time of production is made higher in order to smoothen ferrite carrier particle surfaces having microscopic unevenness. Such a method, however, may cause coalescence between particles to tend to cause faulty images. To cope with this, ferrite carrier particle surfaces having microscopic unevenness are coated with a resin in a large quantity to smoothen carrier particle surfaces to provide the carrier with fluidity and prevent carrier-spent (see, e.g., Japanese Patent Applications Laid-open No. H08-292607 and No. 2003-156887). This method, however, requires the addition of the coat material in a quantity large enough to smoothen the carrier particle surfaces. Hence, the charging of toner may rise so excessively as to provide insufficient image density or cause ground fog. It is also attempted to conversely make the microscopic unevenness present on the carrier particle surfaces by the use of the coat material, to improve charge-providing performance (see, e.g., Japanese Patent Applications Laid-open No. 2002-287431 and No. H10-104884). In this method, however, layers that form the microscopic unevenness may come off as a result of long-term service to cause a problem on durability of the carrier.
Thus, it is sought after to provide a carrier which promises a good rise of charging that is adaptable to the cleanerless system and can maintain a high image quality over a long period of time.
In order to keep toners from changing in charge quantity and to achieve the stabilization of image density, an image forming method is also proposed in which, when the toner consumed as a result of development is replenished, the carrier is replenished together with the toner so that the carrier in a developing assembly can be changed little by little for new one. In such an image forming method as well, it is sought after to provide a carrier which promises a good rise of charging and can maintain a high image quality over a long period of time.